The present invention relates to the measurement of properties of an acoustic source placed in a fluid medium. More particularly, the invention relates to an improved apparatus for measuring the sources's farfield directional or total acoustic power from measurements taken in the nearfield of the source, and where the source may be either fixed or moving.
Up to now, the measurement of the farfield acoustic power of various sound sources has been very difficult to carry out whenever these sources have been fixed. Further, it has been next to impossible to obtain precise measurements when these sources are moving. The most frequently used solution by the prior art to obtain the farfield measurement of the source consists of placing the source in a stationary position in an open space, for example, in nature, and in measuring the farfield pressure with the aid of a microphone which one places at a specified distance from the source. These measurements are ackward and time-consuming if one wishes to obtain precise measurements since they require to operate with the microphone in many points. In addition, these measurements may be distorted due to the presence of other external sound sources.
Other prior-art solutions which produce precise measurements require one to place the source in an anechoic chamber (soundless chamber) which simulates a free acoustic space, or to the degree that one is content with the simple measurement of the total radiated acoustic power radiated without concern for the direction, one usually places the source in a reverberant chamber. The latter prior-art method allowing one to obtain a more rapid and a more precise measurement of the total power. All these methods are not entirely satisfactory, and in general, are time-consuming and require costly investments. More importantly, none of them can be used to obtain measurements of the farfield pressure when the source is moving.
For these reasons, it has already been proposed that one may obtain the total power parameter by direct measurement of the acoustic intensity by means of a pair of microphones which are placed succesively at a large number of points on the surface which surrounds the source or on the source itself. However, such a measuring device does not allow one to determine the direction in which acoustic energy is propagated. In addition, for measurements to be taken specifically under water, one has proposed the use of plane acoustic antennas made up of a plurality of hydrophones placed in a single plane, and which, from measurements taken in the nearfield, permits one to reconstitute the acoustic farfield (Trott Antennas). This technique is specifically described in the article by Anthony J. Rudgers appearing in the Journal of the Acoustical Society of America, Volume 53, No. 5, 1973 beginning at page 1411, and entitled "Determination of the Farfield Radiation of a Noise Source from Nearfield Measurements made with a Trott Array."
The present invention consists of a device which eliminates the disadvantages of the aforementioned devices and methods. Because, by means of measurements taken in the nearfield of the sonorous source placed in a fluid medium, the total and directional acoustic intensity resulting in the farfield may be obtained without the need to hypothesize about the nature of the source.